The present invention provides a rotary positive displacement machine comprising:
a casing having a circular cylindrical internal surface delimiting an operating chamber;
an orbiting piston in the operating chamber, the orbiting piston being mounted so as to orbit about a chamber axis which is the axis of the said internal surface, the orbiting piston having a circular cylindrical external surface, the chamber axis passing through the orbiting piston, a generatrix of the external surface being adjacent to the said internal surface, and a diametrically opposite generatrix being spaced from the said internal surface;
a vane member mounted on the casing, the vane member having a tip face which faces the external surface of the orbiting piston and which has a length substantially equal to that of the orbiting piston; and
a linkage which connects the vane member to the orbiting piston so as to keep the tip face of the vane member adjacent the external surface of the orbiting piston.
In one aspect the invention provides a machine wherein at least one of the said external and internal surfaces is provided with individual compliant strips which are distributed around the said one surface, run parallel to one another, and project above the said one surface.